Do not use Getter interface for Group in Tests
Add options map

The options map is passed to the client Getter function. The client can
then set flags in this map to alter the behavior of the groupcache
system based on the key or data being fetched.
Add the "dynamic" flag

The "dynamic" flag instructs groupcache not to hot cache the retrieved
data, as that data is dynamic and may be removed at some point.

This allows the client to choose which data will be hot cached and
which data will not, dynamically.
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Merge branch 'options'